[2] Its name is in all probability Pre-Greek: it features the ending -sós/-ssós/-ttós, which it shares with many other toponyms in Attica and other rivers in Greece, all of which are considered linguistic substratum survivals.

Its banks—in the busy intersection that presently features the Hilton Hotel and the National Gallery—were grassy and shaded by plane trees, and were considered idyllic in antiquity; they were the favored haunts of Socrates for his walks and teaching.

As urban Athens expanded during the 19th and early 20th centuries, the river became a source of pollution and was converted gradually into a rainwater runoff conduit, covered with streets that track its original, twisting route along the lay of the land.

It then flows to the southeastern flank of the ruined Columns of Olympian Zeus, where it is still visible amidst reed beds, next to the Byzantine chapel of Saint Photeine "of the Ilisos".

The archon in charge would offer goats for sacrifice and the tithe of the sale of war prisoners, while the Athenian youth passed in procession.

Others believe that this is the Shrine of the Nymphs and the river-god Achelous, with a spring of cold water, a plane tree and a willow, where, as Plato writes, Socrates and Phaedros sat during their philosophical chats.